Vapor generating system utilizing fluidized beds

ABSTRACT

A vapor generating system in which a plurality of vertically stacked beds of particulate material containing a solid fuel are disposed in a housing. Pressurized air is passed through each of the fuel beds to fluidize the beds and thus promote the combustion of the fuel and maintain the beds at predetermined temperatures while a heat exchange medium is circulated in a heat exchange relation to the beds. The air is pressurized by an air compressor driven by a gas turbine which, in turn, is driven by the flue gases from the fluidized beds.

Unite States Patent 1 Bryers et a1.

11 1 3,863,606 [4.51 Feb. 4, 1975 VAPOR GENERATING SYSTEM UTILIZING FLUIDIZEI) BEDS [75] Inventors: Richard W. Bryers, Cranford, N.J.;

Jack I). Shenker, Lancaster, Ohio [73] Assignee: The United States of America as represented by the Administrator of the US. Environmental Protection Agency, Washington, DC.

22 Filed: .luly25,1973

211 AppI.No.:382,404

[52] U.S. Cl. 122/4 D, 110/285 [51] Int. Cl. F22b 1/02 [58] Field of Search 122/4 D; 110/285 [56] References Cited UNITED STATES PATENTS 2,818,049 12/1957 Blaskowski 122/4 X 3,431,892 3/1969 Godel 3,648,666 3/1972 Foldes et a1. 3,659,559 5/1972 Foldes et a1. 122/4 3,736,908 6/1973 Ehrlich et al. 122/4 FOREIGN PATENTS OR APPLICATIONS 1,135,508 12/1956 France 122/4 2,060,609 6/1971 Germany 122/4 Primary Examiner- Kenneth W. Sprague Attorney, Agent, or Firm-Marvin A. Naigur; John E. Wilson; Warren B. Kice [57] ABSTRACT A vapor generating system in which a plurality of vertically Stacked beds of particulate: material containing a solid fuel are disposed in a housing. Pressurized air is passed through each of the fuel beds to fluidize the beds and thus promote the combustion of the fuel and maintain the beds at predetermined temperatures while a heat exchange medium is circulated in a heat exchange relation to the beds. The ail" is pressurized I by an air compressor driven by a gas turbine which, in turn, is driven by the flue gasesfrom the fluidized beds.

7 Claims, 4 Drawing Figures VAPOR GENERATING SYSTEM UTILIZING FLUIDIZED BEDS This invention resulted from work done under Contract No. CPA-70-9 with the US. Environmental Protection Agency.

BACKGROUND OF THE INVENTION This invention relates to a vapor generating system, and more particularly, to such a system utilizing a heat exchanger consisting of a plurality of stacked fluidized beds for generating heat.

The use of a low-grade solid fuel, such as coal, is a well-known source of heat in the use of generation of steam. In some of these arrangements the fuel is disposed in a fixed bed, with a chain grate stoker or the like utilized to promote its combustion, and water is passed in a heat exchange relation thereto to produce the steam. However, these arrangements suffer from several disadvantages including problems in handling the solid fuel while adding it to or removing it from the beds during operation. Also, a relatively low heat transfer is achieved and the bed temperatures are often nonuniform and hard to control.

Attempts have been made to produce heat for generating steam by means of a fluidized bed, in which air is passed upwardly through a mass of particulate fuel material causing the material to expand and take on a suspended or fluidized state, with the fluidized bed enjoying the advantages of an improved heat transfer rate, a reduction in corrosion, a reduction in boiler fouling, and the capability of low temperature combustion.

In the past, fluid bed usage has been confined to units operating at atmospheric conditions. It has been found that pressurized fluid bed operation in the vicinity of ten atmospheres or more offers the advantages of a reduction in fluid bed area requirements by virtue of the denser flue gases. This permits operation with deeper beds and a reduction in the total surface area requirements. This results in reduced enclosure shell cost, minimization of costs by increasing the amount of shop fabrication, and reduced construction time. Also, deeper bed configuration promotes improved combustion efficiency and sulfur recovery by virtue of an increased residence time. In this manner, the heat recovery surface is replaced with a more direct conversion I of energy to electricity by means of a gas turbine, and

with the smaller bed size, the number of fuel feed points is greatly reduced, thereby simplifying feeding problems and reducing costs.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a vapor generating system of modular construction including a plurality of stacked fluidized beds which can be manufactured with a minimum of components in a relatively simple manner.

It is a further object of the present invention to provide a vapor generating system which enjoys the advantages of the fluidized bed yet enables several stacked beds of relatively large heights to be utilized.

Toward the fulfillment of these and other objectives, the vapor generating system of the present invention comprises a housing, a means of providing a plurality of vertically spaced beds of particulate fuel material disposed in said housing, a means of passing pressurized air through each of said fuel bedsto promote the combustion of said fuel and maintain said compartments at predetermined temperatures, and a means for successively passing a heat exchange medium in a heat exchange relation to said fluidized beds.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial sectional partial schematic view of the vapor generator utilized in the system of the present invention;

FIG. 2 is a partial schematic partial skeleton view depicting a portion of the vapor generator of FIG. 1;

FIG. 3 is a cross-sectionalview taken along the line 3-3 of FIG. 1 with a portion of the piping being omitted; and

FIG. 4 is a schematic view depicting the system ofthe present invention.

DESCRIPTION OF TH E PREFERRED EMBODIMENTS Referring specifically to FIG. 1 of the drawings, the reference numeral 10 refers in general to a housing having several openings for the passage of ducting and tubing for purposes that will be described in detail later. A heat exchanger 12 is disposed within the housing and is formed by a front wall 14, a rear wall 16, an an intermediate wall 18. A pair of sidewalls, omitted from FIG. 1 for the convenience of presentation, but shown by the reference numerals 20 and 22 in FIG. 2, extend from the front wall 14 to the rear wall 16. Each of the walls are formed by a plurality of finned tubes welded together in a conventional manner and extending for the entire length of the wall.

A plurality of horizontal perforated air distribution plates 24 are disposed in a spaced relation in the area defined by the walls l4, 18, 20 and 22 to divide the area into a plurality of vertically stacked compartments which define four beds, 26a, 26b, 26c, and 26d. It is noted that FIG. 1 is not to scale since, in actual practice the vertical heights of each bed is much greater relative to its horizontal depth than shown.

An air plenum chamber is located immediately below each bed 26a26d for receiving air through a pair of dampers 28 and passing the air through the distribution plates 24 and into the beds for fluidizing the beds.

Particulate fuel from a source such as a pneumatic feeder or the like is fed into the beds 26via a plurality of pipes 29 extending through suitable openings provided in the walls 16 and 18 and the housing 10. Although only-one pipe 29 is shown in FIG. 1 associated with each bed, it is understood that, in actual practice, a multiplicity of such pipes are associated with each bed and several extend through the wall 18 and wall 14.

According to a preferred embodiment, the particulate fuel is in the form of a mixture of crushed bituminous coal and limestone, with the latter functioning as a sorbent for the sulfur released during combustion of the coal in accordance with conventional practice. Since the low combustion temperatures and the low excess air requirements also reduce the nitrogen oxide from the combustion gas, the latter contains a minimum of pollutants.

Air, which is pressurized by an external unit in a manner to be described later, is passed into the housing 10 through an inlet 30 whereby it passes into the space between the housing 10, and the heat exchanger 12 and divides up into separate paths before passing into the dampers 28 associated with each bed 26a-26d to fluidize the beds in a conventional manner. It is understood that the velocity and rate of flow of the air passing through the beds is regulated so that it is high enough to fluidize the particulate fuel in the beds to obtain economical burning or heat release rates per unit area of bed, while being low enough to avoid the loss of too many fine fuel particles from the bed and to allow sufficient residence time of gases for good sulphur removal by the absorbent added to the fuel.

The hot gases from the fluidized beds 26a-26d exit through outlets 32 extending through the intermediate wall 18, and mix in the area between the walls 16 and 18 before passing outwardly through an outlet 34 registering with the wall 16.

A plurality of headers, downcomers and associated piping are provided to selectively direct water through the tubes of the walls 14,16, 18, 20 and 22 and the beds 26 to gradually heat the water in predetermined increments. In particular, and referring specifically to FIG. 2, a header is shown in general by the reference numeral 36, and is formed by three pipes connected in a substantially U shape, with the base portion thereof extending coplaner with, and immediately above, the wall 16. A header 38, in the form of a straight conduit, is disposed immediately above the wall 18. A pair of substantially L-shaped headers 40 and 42 are also provided, with one leg portion of the header 40 being coplaner with a portion of the wall 20 and the other leg portion of the header being coplaner with a portion of the wall 14; while one leg portion of the header 42 is coplaner with the other portion of the wall 14 and the other leg portion of the header 42 is coplaner with a portion of the wall 22.

In a similar manner, a substantially U-shaped header 44 is provided at the bottom portion of the heat exchanger 12, with the base portion thereof being substantially coplaner with, and immediately below, the wall 16. A header 46 is positioned at the end ofthe wall 18 and extends coplaner therewith, while a pair of L- shaped headers 48 and 50, substantially corresponding in shape to the headers 40 and 42, are disposed in corresponding positions at the bottom end portions of the walls 14, 20, and 22. Although the various headers are shown in FIG. 2 as being formed of regular piping, it is understood that a plurality of spaced openings are provided through each pipe which register with the finned tubes forming the corresponding walls.

A plurality of downcomers, or vertically extending conduits, 52, 54, 55, 56, and 58 are provided and are connected by means ofa plurality of feeder tubes to the various headers for routing the water and water steam through various portions of the walls 14, 16, and 18. In particular, and referring to FIG. 1, water is initially introduced via an inlet conduit 60 into a header 62 whereby it is distributed to a plurality of tubes completely submersed in the lowermost bed 26a. Although not clear from the drawings, the latter tubes are spaced across the width of the heat exchanger, with each extending in a serpentine relationship, to form a tube bank or bundle shown by the reference numeral 64 with. of course, only an outermost tube being shown in FIG. 1. The water is partially vaporized in the uppermost loops of the tube bundle 64 and the liquid-steam mixture is collected in a header 65 and passed by a feeder tube 66 to the downcomer 56 whereby it is routed downwardly to the header 44 in communication with the finned tubes forming the wall 16 and a portion of the walls 20 and 22 as better shown in FIG. 2. The

liquid-steam mixture is then routed upwardly through the wall 16 and the above portions of walls 20 and 22 to the header 36. From the latter header the mixture is passed through a plurality of feeder tubes 68 to the downcomer 52 whereby it passes downwardly and into the header 46 via a feeder tube 70. The header 46 then distributes the mixture to the finned tubes of the wall 18 whereby it is routed upwardly through the latter wall to the header 38 and from the latter, via the feeder tube 72 to the downcomer 58.

From the downcomer 58 the mixture passes through feeder tubes 74, to the header 50 which distributes the mixture to the corner defined by the adjoining sections of the wall 14 and the wall 22. The mixture then travels upwardly through the latter wall sections to the header 42 and, through feeder tubes 76, to the downcomer 54 for another downward pass. From the downcomer 54, the mixture is passed, via a feeder tube 78, to the header 48 which distributes the mixture to the corner defined by the adjoining portions of the walls 14 and 20. The mixture then passes upwardly through the latter portions of the walls 14 and-20 to the header 40 and from the latter, via feeder tubes 80, to the downcomer 55. It is noted that FIGS. 1 and 2 are somewhat inconsistent with respect to the location of several of the above described feeder tubes. This is done to better clarify the presentation, especially since the particular location of the feeder tubes form no portion of the present invention.

It is noted that. since the downcomers discussed above are not heated, all movement of the liquid-steam mixture through the heated walls in accordance with the foregoing is upward. This minimizes separation of the liquid and vapor and thereby greatly reduces thermal fatigue of the tubes due to alternate wetting and drying of the tubes by an otherwise heterogeneous mixture.

The downcomer 55 connects with a header 82, shown in FIG. 1, which directs the steam into a tube bundle 84 submerged in the bed 26b, and from the latter, through a feeder tube-header unit 86 into a tube bundle 88 immersed in the bed 260. This raises the temperature of the steam to superheat before it passesinto a header 90 and exits to a steam turbine or the like through an outlet conduit 92.

A tube bundle 94 is provided in the uppermost bed 26d to receive relatively low temperature steam which has previously been used in another stage of the plant such as the above steam turbine, to raise its temperature for further use. The latter steam is received by an inlet conduit 96 and passed, via a header 98, through the tube bundle 94 to raise the temperature of the steam before it exits via a header 100 and an outlet 102.

An additional bed, in the form of a carbon burnup cell 104 extends in the space between the lower portions of the walls 16 and 18. This cell 104 is filled with fuel material from the primary fluid beds which has been elutriated and collected in a manner to be described in detail later, and passed into the cell through an inlet 106, for further combustion. The cell 104 has an air gas circuit, separate from that of the primary circuit, in the form of an inlet 108 through which air passes into a damper 109 before passing through the bed and exiting through a gas outlet 110.

FIG. 4 schematically depicts the flow patterns of the air and gases in the vapor generating system of the present invention. In particular, the heated gases from the gas outlet 34 and the carbon burnup cell outlet 110 are combined and are passed to a cyclone type dust collector 120 which removes the fine fuel particles entrained in the gases and directs the particles to a dust collector 122 and then into an injector 124 which injects the fines, through the inlet ")6, back into the carbon burnup cell 104 for further burning.

The air from the collector 120, which is relatively free from the fine fuel particles, is passed into the inlet side of a gas turbine 126 operatively connected to an air compressor 128 which receives atmospheric air and pressurizes it before passing it to the primary air inlet 30 of the housing 10.

The gas turbine 126 also drives a generator 130 in a conventional manner which provides power auxiliary to that provided by the main turbine to which steam is fed through the outlet 92 as described earlier.

In operation, compressed air from the air compressor 128 enters the housing through the inlet 30 and fills in the void between the tube walls of the heat exchanger 12 and the housing 10. This volume acts as a manifold to distribute the air into the beds 26a-26d through their respective air dampers 28. The particulate fuel in each bed is ignited and, by virtue of being fluidized by the air, effects a relatively high bed-to-tube heat transfer coefficient. The flue gases from the respective beds then pass outwardly through the outlets 32 and then, through the outlet 34, into the cyclone 120 for separation and further treatment as described above.

The water to be vaporized enters the vapor generator 12 in the bottom of the bed 26a via the inlet 60 and passes through the tube bank 64 before being routed through the various portions of the walls 14, 16, 18, 20, and 22 via the headers and feeder tubes as discussed above, to completely vaporize the water. From the last wall portion the steam is then passed into the superheater banks 84 and 88 whereby its temperature is raised to superheat before it is collected in the header 90 and passed, through the outlet 92, into a steam turbine or the like (not shown) for driving same.

There are many advantages of the arrangement of. the present invention. For example, the use of the vertical stacked compartments defined by continuous walls considerably reduces the manufacturing costs and time, since it minimizes headers, interconnecting piping, and downcomers yet permits a maximum use of the heat transfer surfaces involved. Of course, the free movement of the particulate fuel in the fluidized bed promotes rapid heat transfer both within the bed and between the bed and the submerged tube banks. As a result, bed temperatures are uniform and easy to control.

The cost of construction is reduced by minimizing boiler cross sectional area and maximizing the number of components that are shop fabricated, so that the boiler dimensions can meet shipping, dimensional and weight limitations. Also, the start-up procedures are greatly simplified by assigning only one heating function to each bed, such that no bed must be started with uncooled tubes. Thus, the evaporating beds are started first with circulating water, and superheating beds are started last after steam has been generating. The assigning of separate heating functions to the individual beds also simplifies and improves steam temperature control by differential firing of coal to each bed. The modular construction simplifies load control, and a four to one turn down can be achieved by simply shutting down three modules. This also improves on load time as individual modules can be serviced without loosing the entire boiler system.

It should also be noted that a vertically stacked bed with a fin-tube water wall construction simplified the circuitry and minimizes the number of headers, downcomers and feeder pipes. Thus, the fin-tube water wall construction has the following attendant advantages:

a. provides a support for the heat transfer surface, pressure parts and fluid beds;

b. protects the enclosure wall or shell from high tempcrature gases;

c. provides a heat transfer surface thereby maximizing the utility of all components to serve their pri mary function of heat exchange;

d. provides a partition between the flue gas and inlet air;

e. reduces the surface requirements in the bed;

f. reduces costs by employing shop fabrication techniques; and

g. reduces maintenance cost.

Still other advantages of the heat exchanger of the present invention include reduction in the corrosion of the tubes, etc., due to the relatively low combustion temperatures available and a reduction in costs since cheaper construction materials can be used by virtue of the high heat transfer rates at the relatively low temperatures.

The use of pressurized air for fluidizing the beds leads to still further advantages. For example, the vertical height of each bed can be increased, thus permitting a corresponding decrease in the width and length of the beds. This permits maximum shop fabrication, i.e., a reduction in cost as a result of adhering to optimum shipping dimensions, and shell costs are also reduced by fabricating the pressurized vessel to a relatively smaller diameter. The mechanical problems, and therefore costs, normally associated with feeding the coal into the beds is also reduced.

It is understood that the compactness and operation of the heat exchanger lends itself to incorporation in a modular system, since four or five units described above can be utilized in a side by side relation.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features ofthe invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.

What is claimed is:

1. A vapor generating system comprising a housing, means defining a plurality of vertically aligned beds of particulate fuel material in said housing, a source of pressurized air, means to pass said pressurized air through each of said beds to promote the combustion of said fuel and maintain said beds at predetermined temperatures, a plurality of vertically disposed tubes connected together to form the walls of said housing for passing a heat exchange medium in a heat exchange relation to said beds, and piping means for directing said medium into the lower ends of a portion of said tubes for passage upwardly to the upper ends thereof and from the upper ends of said. tubes downwardly externally of said tubes and back into another portion of said tubes for passage upwardly therethrough.

2. A vapor generating system comprising a housing, means defining a plurality of vertically aligned beds of particulate fuel material in said housing, a source of pressurized air, means to pass said pressurized air through each of said beds to promote the combustion of said fuel and maintain said beds at predetermined temperatures, a plurality of vertically disposed tubes connected together to form the walls of said housing for passing a heat exchange medium in a heat exchange relation to said beds, and piping means for directing said medium through at least one of said beds and then into the lower ends of a portion of said tubes for passage upwardly to the upper ends thereof.

3. A vapor generating system comprising a housing, means defining a plurality of vertically aligned beds of particulate fuel material in said housing, a source of pressurized air, means to pass said pressurized air through each of said beds to promote the combustion of said fuel and maintain said beds at predetermined temperatures, a plurality of vertically disposed tubes connected together to form the walls of said housing for passing a heat exchange medium in a heat exchange relation to said beds, and piping means for directing said medium into the lower ends of a portion of said tubes for passage upwardly to the upper ends thereof and then through at least one of said beds.

4. The system of claim 3 wherein said medium is water and wherein the temperatures of said beds are such that said water is vaporized as a result of passing through said tubes and the temperature thereof is raised to superheat as a result of passing through said beds.

5. The system ofclaim 1 wherein said source of pressurized air is an air compressor and further comprising a gas turbine operatively connected to said compressor, and means for directing the gases from said beds to said turbine for driving said turbine and therefore said compressor.

6. The system of claim 2 wherein said source of pressurized air is an air compressor and further comprising a gas turbine operatively connected to said compressor,

- and means for directing the gases from said beds to said turbine for driving said turbine and therefore said compressor.

7. The system of claim 3 wherein said source of pressurized air is an air compressor and further comprising a gas turbine operatively connected to said compressor, and means for directing the gases from said beds to said turbine for driving said turbine and therefore said compressor. 

1. A vapor generating system comprising a housing, means defining a plurality of vertically aligned beds of particulate fuel material in said housing, a source of pressurized air, means to pass said pressurized air through each of said beds to promote the combustion of said fuel and maintain said beds at predetermined temperatures, a plurality of vertically disposed tubes connected together to form the walls of said housing for passing a heat exchange medium in a heat exchange relation to said beds, and piping means for directing said medium into the lower ends of a portion of said tubes for passage upwardly to the upper ends thereof and from the upper ends of said tubes downwardly externally of said tubes and back into another portion of said tubes for passage upwardly therethrough.
 2. A vapor generating system comprising a housing, means defining a plurality of vertically aligned beds of particulate fuel material in said housing, a source of pressurized air, means to pass said pressurized air through each of said beds to promote the combustion of said fuel and maintain said beds at predetermined temperatures, a plurality of vertically disposed tubes connected together to form the walls of said housing for passing a heat exchange medium in a heat exchange relation to said beds, and piping means for directing said medium through at least one of said beds and then into the lower ends of a portion of said tubes for passage upwardly to the upper ends thereof.
 3. A vapor generating system comprising a housing, means defining a plurality of vertically aligned beds of particulate fuel material in said housing, a source of pressurized air, means to pass said pressurized air through each of said beds to promote the combustion of said fuel and maintain said beds at predetermined temperatures, a plurality of vertically disposed tubes connected together to form the walls of said housing for passing a heat exchange medium in a heat exchange relation to said beds, and piping means for directing said medium into the lower ends of a portion of said tubes for passage upwardly to the upper ends thereof and then through at least one of said beds.
 4. The system of claim 3 wherein said medium is water and wherein the temperatures of said beds are such that said water is vaporized as a result of passing through said tubes and the temperature thereof is raised to superheat as a result of passing through said beds.
 5. The system of claim 1 wherein said source of pressurized air is an air compressor and further comprising a gas turbine operatively connected to said compressor, and means for directing the gases from said beds to said turbine for driving said turbine and therefore said compressor.
 6. The system of claim 2 wherein said source of pressurized air is an air compressor and further comprising a gas turbine operatively connected to said compressor, and means for directing the gases from said beds to said turbine for driving said turbine and therefore said compressor.
 7. The system of claim 3 wherein said source of pressurized air Is an air compressor and further comprising a gas turbine operatively connected to said compressor, and means for directing the gases from said beds to said turbine for driving said turbine and therefore said compressor. 